Diplexers are commonly used to split an incoming signal into two component parts, respectively having differing frequency components. The frequency splitting properties of diplexers are often used in the communications field, and particularly in radars to switch between two different operating frequencies.
Previously used diplexers operating in the microwave frequency range possessed a relatively narrow bandwidth (e.g. 8-12 GHz) and were therefore unsuitable for use in applications where is was necessary to cover a relatively wide bandwidth, e.g. 8-18 GHz. Consequently, in the past, in order to split an incoming wideband microwave signal into upper and lower components, it was necessary to use a switch in order to switch between the upper and lower frequency bands. The bandwidth restrictions for previous diplexers is a result of the inherent limitations of the ordinary waveguides used in such diplexers. So-called "ridged" waveguides and tapered waveguides have been used in the past in microwave frequency applications, but never have been combined for use in a balanced diplexer to increase bandwidth and achieve a sharp frequency cut-off.